Synthesis of oxalate esters by catalytic oxidative carbonylation of borate esters

ABSTRACT

A process for the preparation of oxalate esters by the catalytic oxidative carbonylation of orthoborate, metaborate and polyborate esters or mixtures thereof with carbon monoxide and oxygen or an oxygen-containing gas in the presence of a metal salt catalyst, an amine base and at least a catalytic amount of an alcohol. Preferably a catalytic amount of particular metal oxidizing salts are employed along with a catalytic amount of an amine salt compound which may be formed in situ by the addition of an acid. Alternatively various counterions and ligands of the metal salt catalysts may be employed.

BACKGROUND OF THE INVENTION

A number of prior art processes have been proposed for the preparationof oxalate esters by the oxidative carbonylation of alcohols in thepresence of metal salt catalysts, dehydrating agents, ferric or cupricredox agents in solution and various reaction accelerators.

The present invention is directed to a process for the preparation ofoxalate esters while avoiding the problems associated with the prior artprocesses of carbonylating alcohols to obtain the desired oxalate ester.More particularly, the present process relates to the synthesis ofoxalates by reacting carbon monoxide and oxygen with an orthoborateester under elevated temperature and pressure conditions in the presenceof a catalytic amount of a palladium, platinum, or rhodium salt catalystand at least a catalytic amount of an amine base, and an alcohol andincludes the employment of catalytic amounts of copper (II) or iron(III) oxidant salts in addition to catalytic amounts of an ammonium orsubstituted ammonium salt compound and ligands of the metal saltcatalysts.

U.S. Pat. No. 3,393,136 describes a process for the preparation ofoxalates by contacting carbon monoxide at superatmospheric pressure,with a saturated monohydric alcohol solution of a platinum group metalsalt and a soluble ferric or cupric salt (redox agent) while maintainingthe salts in a highly oxidized state by the simultaneous introduction ofoxygen or the application of a direct current electrical potential tothe reaction zone. When oxygen is employed, explosive mixtures of oxygenand combustible organic vapors in the gas phase must be avoided andwater scavengers or dehydrating agents such as alkyl orthoformic acidesters must be added to the liquid phase to prevent the accumulation ofwater.

In a recent article by Donald M. Fenton and Paul J. Steinwand, Journalof Organic Chemistry, Vol. 39, No. 5, 1974, pp. 701-704, a generalmechanism for the oxidative carbonylation of alcohols to yield dialkyloxalates using a palladium redox system, oxygen and dehydrating agentshas been proposed. In the absence of the necessary dehydrating agent, alarge amount of carbon dioxide is formed and oxalates are not produced.The necessity of the iron or copper redox system during the oxalatesynthesis is emphasized.

A recent West German Pat. No. 2,213,435 discloses a method for thesynthesis of oxalic acid and oxalate esters in water and alcoholrespectively. A platinum group metal salt, a salt of a metal moreelectropositive than the platinum group metal, e.g. copper (II) chlorideand an alkali metal salt comprise the catalyst. Oxygen in stoichiometricamounts was employed as the oxidant. A disadvantage of such reaction isthat explosive mixtures of oxygen and carbon monoxide are necessary toeffect reaction. Under non-explosive conditions only trace amounts ofoxalate can be obtained.

A more recent West German Patent Publication No. 2,514,685 discloses aprocess for the preparation of dialkyl oxalates by reacting an aliphaticalcohol with carbon monoxide and oxygen under pressure in the presenceof a catalyst of a mixture of a salt of a metal from the platinum groupand a salt of copper or iron and a reaction accelerator includingnitrates, sulfates, bicarbonates, carbonates, tertiary amines andhydroxides and carbonates of alkali metals and alkaline earth metals,pyridine, quinoline, urea and thiourea. Alcohol conversions are low.

The oxalate products of this invention may be employed, for example, ascellulose ether or ester and resin solvents, as dye intermediates and inthe preparation of pharmaceuticals.

The process of the present invention provides a method of carrying outthe oxidative carbonylation of orthoborate esters to produce an oxalateester essentially without the coproduction of water which acts to poisonthe catalyst system and which even in small amounts also causes theproduction of large quantities of carbon dioxide and an attendant lossof the desired oxalate ester. Thus, by the process of the presentinvention, only small concentrations of water can accumulate in thereaction system since by the mechanism of the reaction any water whichmight be formed is rapidly consumed upon formation of coproductboroxines or polyborate compounds. In addition, the coproduction ofcarbonate esters associated with such carbonylation reactions areminimized giving excellent selectivities to oxalate esters with highconversions of the borate ester. The boroxine or polyborate estercoproduced with the desired oxalate ester by the oxidative carbonylationreaction of the orthoborate ester may be readily separated from thedesired oxalate and converted back to the respective reactant.

Other advantages of the present invention, as compared to known priorart processes for the production of oxalates are (1) elimination ofhazardous operational conditions by avoiding explosive mixtures ofoxygen and carbon monoxide, (2) avoiding the use of large amounts ofcorrosive chloride ions and (3) the ease of recovery and regeneration ofthe metal salt catalysts for reuse in the process.

SUMMARY OF THE INVENTION

According to the present invention there is provided a catalyticoxidative carbonylation process for the preparation of oxalate esters byreacting at least stoichiometric quantities of carbon monoxide andoxygen with an orthoborate ester, which process is carried out atelevated temperatures and pressures in the presence of a metal saltcatalyst and a catalytic amount of an amine base and an alcohol andunder relatively anhydrous conditions. The process of the invention alsopreferably employs, in catalytic amounts, particular metal oxidant saltsand an acid or an ammonium or substituted ammonium salt compound toprovide a pronounced effect on oxalate ester selectivity, and highconversions to the oxalates over the carbonates which may be present inonly trace amounts. In addition, it has been found that alternativelycatalytic amounts of various ligands, which will not work in themselves,may be used as co-catalysts in conjunction with the metal saltcatalysts, the amines, the amine salts and the oxidant salts.

It is a primary object of this invention to provide a process for thepreparation of oxalate esters while avoiding operational problemsassociated with prior processes.

It is another object of this invention to provide a novel reactionsystem useful in the conversion of carbon monoxide, oxygen, andorthoborate esters to oxalate esters.

It is a further object of this invention to provide a specific mechanismfor the employment of catalysts, oxidant salts, amine salts and aminesin an oxidative carbonylation process employing orthoborate esters asreactants.

These and other objects and advantages of this invention will becomeapparent from the description of the invention which follows and fromthe claims.

DESCRIPTION OF THE INVENTION

In accordance with the invention, an oxalate ester is produced byreacting, under relatively anhydrous liquid phase conditions, anorthoborate ester with carbon monoxide and oxygen at elevatedtemperatures and pressures in the presence of a catalyst comprising apalladium, rhodium or platinum metal salt or mixtures thereof, with orwithout a ligand such as lithium iodide as a co-catalyst, and incatalytic amounts, a base of a primary, secondary or tertiary amine, andan alcohol and preferably, in catalytic amounts, a copper (II) or iron(III) metal oxidant salt, an ammonium salt or amine salt or acidstronger than water which will not complex too strongly with the metalsalt catalyst. The synthesis of the oxalate esters is carried outaccording to the following postulated equation ##STR1## wherein R, R'and R" are substituted or unsubstituted alkyl or aralkyl groups. R, R'and R" may be the same or different and may contain other substituentssuch as amido, alkoxy, amino, carboxy, cyano, etc. radicals. Thesubstituents, in general, do not interfere with the reaction of theinvention. Boroxines having the formula ##STR2## and polyborates havingthe formula (ROBO).sub. 3.× B₂ O₃ may also be employed as reactantseither alone or in admixture with the orthoborates to produce oxalateesters by the process of the invention.

As indicated above, catalytic amounts of an amine as a base, and analcohol are added to the reaction mixture and preferably in addition incatalytic amounts a metal oxidant salt and an amine salt. The amine saltso added may also be formed in situ in the reaction mixture by theaddition of an acid such as sulfuric acid in order to form the necessaryquantity of amine salt. Thus, for example, triethylamine can be employedinitially in sufficient amounts and sulfuric acid added to formtriethylammonium sulfate in the desired catalytic quantities. Theaddition of the amine salt helps to maintain the proton acidity of thereaction system.

The reaction between the orthoborate ester, carbon monoxide and oxygenmay be carried out in an autoclave or any other high pressure reactor.Although the order of addition of reactants and other components mayvary, a general procedure is to charge the orthoborate ester, amine,alcohol, amine salt (or the required amount of amine and acid),catalyst, and the oxidant salt into the reaction vessel, introduce theproper amount of carbon monoxide and oxygen to the desired reactionpressure and then heat the mixture to the desired temperature for theappropriate period. The reaction can be carried out batchwise or as acontinuous process and the order of addition of the reactants may bevaried to suit the particular apparatus employed. The reaction productsare recovered and treated by any conventional method such asdistillation and/or filtration, etc. to effect separation of the oxalatefrom unreacted materials, catalyst, oxidant salt, amine salt, byproducts, etc.

The orthoborate esters employed in at least stoichiometric quantitiesand suitable for use in the process of the present invention conform tothe general formula ##STR3## respectively as indicated hereinabove. R,R' and R" which may be the same or different may be substituted orunsubstituted alkyl or aralkyl groups preferably containing from 1 to 10carbon atoms in the alkyl chain and from 1 to 2 aryl group substituentswhen R, R' and R" or all three is an aralkyl group. Particularlypreferred are the orthoborates wherein R, R' and R" are straight chainalkyl groups containing from 1 to 8 carbon atoms such as triethylborate.

Representative orthoborate esters suitable for use in this inventioninclude, for example, trimethyl borate, triethyl borate,tri-2-chloroethyl borate, tritolyl borate, tri-methoxybenzyl borate,tri-chlorobenzyl borate, tri-benzyl borate, tri-4-butylphenyl borate,tri-n-propyl and tri-isopropyl borates, tri-(1,3-dichloro-2-propyl)borate, tri-n-butyl, tri-s-butyl and tri-t-butyl borates, tri-(β,β,β-trichloro-t-butyl) borate, triphenyl borate, tri-o-chlorophenyl borate,tri-n-amyl borate, tri-t-amyl borate, tri-(o-phenylphenyl) borate,tri-n-hexyl borate, tri-3-heptyl borate, tri-3-pentyl borate,tri-n-octyl and tri-isooctyl borates, tri-(2-ethylhexyl) borate,tri-(methylisobutylcarbinyl) borate, tri-(diisobutylcarbinyl) borate,tri-(2,5-dimethylbenzyl) borate, etc.

The alcohols employed in catalytic quantities and suitable for use inthe process of the present invention can be monohydric saturatedaliphatic and alicyclic alcohols or aromatic alcohols and may containother substituents such as amido, alkoxy, amino, carboxy, cyano, etc.radicals in addition to the hydroxyl group. The substituents, ingeneral, do not interfere with the reaction of the invention.

The alcohols which may be employed in concentrations of from 0.1 to 50weight percent preferably from 1 to 3 weight percent and which may beprimary, secondary or tertiary alcohols conform to the general formulaROH, wherein R is an optionally substituted aliphatic or alicyclic groupcontaining from 1 to 20 carbon atoms and preferably unsubstitutedaliphatic alcohols containing from 1 to 8 carbon atoms. R may also be anaromatic group containing one or more benzenoid rings preferably notmore than 3 rings which may be fused or joined by single valency bonds,directly or through bridging groups which may be, for example, oxygen orsulfur atoms or sulfoxide, sulfone or carbonyl groups or alkylene groupsin which, if desired, the carbon chain may be interrupted by, forexample, oxygen or sulfur atoms, sulfoxide, sulfone or carbonyl groups,for example methylene, oxymethylene, dimethylene sulfone or dimethyleneketone groups. Representative alcohols especially suitable for use inthis invention are monohydric alcohols such as methyl, ethyl, n-, iso-,sec-, and tert-butyl, amyl, hexyl, octyl, lauryl, n- and iso-propyl,cetyl, benzyl, chlorobenzyl and methoxy-benzyl alcohols as well as, forexample cyclohexanol, octanols, heptanols, decanols, undecanols, 2-ethylhexanol, nonanol, myristyl alcohol, stearyl alcohol, methylcyclohexanol, pentadecanol, oleyl and eicosonyl alcohols, and the like.The preferred alcohols are the primary and secondary monohydricalcohols, such as methanol, ethanol and 1- and 2-propanol, n-butylalcohol etc. up to 8 carbon atoms.

The amines employed in catalytic quantities in the process of theinvention, used as a base, may be primary, secondary or tertiary aminesand include aliphatic, cycloaliphatic, aromatic and heterocyclic aminesor mixtures thereof. The amines may be unsubstituted or contain othersubstituents such as halides, alkyl, aryl, hydroxy, amino, alkylamino,carboxy, etc. The amines may be employed in the reaction inconcentrations of from 0.1 to 5 weight percent and preferably at aconcentration ˜3 weight percent.

Representative amines as hereinabove described, include for example,mono-, di- and tri-methyl, ethyl and propyl amines, iso- anddiisopropylamines, allyl amines, mono-, di-, tri-, iso and diisobutylamines, 1-methylpropyl amine, 1,1-dimethylethyl amine, amyl amines,cyclohexyl amine, dicyclohexylamine, 1,3-dimethyl-butyl amine,2-ethylhexylamine, 1-cyclopentyl-2-amino propane,1,1,3-tetramethylbutylamine, aniline, ethylene diamine, methylenediamines, ethanolamine, octylamines, n-decylamine, do-, tetra-, hexa-,octa-, dido-, ditetra-, diocta-, trido- and triocta-decylamines,chloro-anilines, nitro-anilines, toluidines, naphthylamine, N-methyl andN-ethyl and N,N-dimethyl and N,N-diethyl aniline, di- andtri-phenylamines, N,N-diamylaniline, benzyl dimethyl amine, piperidine,pyrrolidine, etc. The preferred amines used as bases are the tertiaryamines such as triethylamine.

The metal salt catalysts which may be employed in the process of thisinvention are the palladium, platinum and rhodium salts. Among thechemical forms of the metal compounds which can be used are thepalladium, platinum and rhodium, halides, sulfates, oxalates andacetates preferably the palladium (II) halides such as palladium (II)chloride and palladium (II) iodide. Representative catalytic metal saltcompounds include, for example palladium (II) chloride, rhodium (III)chloride, palladium (II) sulfate, palladium (II) oxalate, palladium (II)acetate, palladium (II) iodide, rhodium (III) bromide, platinum (II)chloride, platinum (II) sulfate, etc.

The catalysts employed may be in a homogeneous state in the reactionmixture at reaction conditions. Thus, the catalysts may be present insolution, or suspension and may also be on support materials such asalumina, silica gel, aluminosilicates, activated carbon or zeolites.

The reaction is generally carried out in the presence of a catalyticproportion of the metal salt catalyst and will proceed with smallamounts of the metal salt catalyst compounds hereinabove described.Generally the proportions of the metal salt catalyst used in thereaction will be equivalent to between about 0.001 to 5 weight percentof the orthoborate employed and are preferably employed in amountsbetween about 0.01 to 2 percent by weight of the orthoborate employed.Larger or smaller amounts may be employed at varied pressures andtemperatures.

As mentioned hereinabove, a ligand or coordination complex compound ofthe metal catalyst may be employed in the process of the invention as aco-catalyst and thereby also achieve a pronounced increase in theselectivity for the oxalate ester. The ligands may be, for example,alkyl or aryl phosphines, arsines, iodides or stibines. The complexes ofthe metal catalysts which are suitable as co-catalysts in the process ofthe present invention include complex compounds of palladium, platinumand rhodium. The complex compounds may contain one or more atoms of thesaid metals in the molecule and when more than one such atom is present,the metals may be the same or different. The mono- or poly-dentateligands which are present in the molecule of the complex compounds andin which at least one of the electron-donating atoms is an atom ofphosphorous, arsenic or antimony or an iodide ion containing a lone pairof electrons may be, for example, organo-phosphines, -iodides, -arsinesand -stibines. Suitable mono-dentate ligands include alkyl phosphinessuch as trimethylphosphine and tributylphosphine, aryl-phosphines suchas diethylphenyl-phosphine and radicals derived from such phosphines,for example the radical having the formula -- P(CH₃)₂. Hydrocarbyloxyphosphines, i.e., phosphites, such as triphenyl phosphite may also beemployed. Suitable polydentate ligands include tetramethyldiphosphinoethane and tetraphenyl diphosphinoethane. Exactly analogousderivatives of arsenic and antimony may be used; however, because oftheir greater ease of preparation and stability of the derivedcomplexes, the hydrocarbyl derivatives of phosphorus are preferred. Itis also preferred to employ alkali metal iodides, e.g. lithium iodide.

The complex compounds suitable for use in the process of the presentinvention may contain in the molecule, in addition to the ligandsdiscussed above, one or more other atoms, groups or molecules, which arechemically bonded to the metal atom or atoms. Atoms which may be bondedto the metal include, for example, hydrogen, nitrogen and halogen atoms;groups which may be bonded to the metal include, for examplehydrocarbyl, hydrocarbyloxy, carbonyl, nitrosyl, cyano and SnCl₃ --groups; molecules which may be bonded to the metal include, for example,organic isocyanides and isothiocyanates.

Examples of suitable complex compounds are those represented by thefollowing formulae:

    ______________________________________                                        RhBr.sub.3 (PPhEt.sub.2).sub.3                                                                   Rh(CO)Cl(AsEt.sub.3).sub.2                                 RhCl(CO)(PPhEt.sub.2).sub.2                                                                      RhCl(CO)(PEt.sub.3).sub.2                                  Rh(Ph.sub.2 PCH.sub.2 CH.sub.2 PPH.sub.2).sub.2 Cl                                               PdCl.sub. 2 (PPh.sub.3).sub.2                              Rh[(PhO).sub.3 P].sub.3 Cl                                                                       Li.sub.2 PdI.sub.4                                         PdI.sub.2 (PPh.sub.3).sub.2                                                                      PtCl.sub.2 (p-ClC.sub.6 H.sub.4 Pn-Bu.sub.2).sub.2         ______________________________________                                    

The complex compounds employed may be introduced into the reactionmixture as such, or they may be formed in situ from a suitable metalcompound noted above and the desired ligand.

The ligand or complex compounds may be used in catalytic amounts of from0 to 3 percent preferably from 0.1 to 1 percent by weight of theorthoborate ester to be reacted although larger or smaller amounts maybe employed at varied pressures or reaction rates.

The oxidizing salts which may be employed in an anhydrous condition andin catalytic amounts of from 0 to 10 weight percent preferably 3 to 5weight percent in the process of the invention include the copper (II)salts such as the sulfates, trifluoroacetates, oxalates, or acetatespreferably the copper (II) sulfates and trifluoroacetates.Representative oxidant salts include, for example, copper (II) sulfate,copper (II) trifluoroacetate, copper (II) acetate, copper (II) oxalate,copper (II) triflate and copper (II) fluorosulfonate. Excess halides aredetrimental to the reaction system of the present invention.

The amine salts which are employed in an anhydrous condition and in acatalytic amount of from 0 to 10 weight percent preferably in aconcentration ˜ 10 weight percent in the process of the inventioninclude, for example, the ammonium and substituted ammonium sulfates,trifluoroacetates, and acetates, preferably the tertiary amine sulfatessuch as triethyl ammonium sulfate. Representative amine salts include,for example diethylammonium sulfate, ethylammonium sulfate,butylammonium sulfate, ammonium sulfate, trimethylammonium sulfate,mono-methylammonium sulfate, trimethyl ammonium hydrogen sulfate,ammonium acetate, ammonium trifluoroacetate, methyl-, ethyl- andbutylammoniumtrifluoroacetate, etc.

The amine salts may be added as such or formed in situ in the requiredamounts upon the addition of an acid, such as, sulfuric, benzenesulfonic, phosphoric, o-boric, p-toluene sulfonic, acetic ortrifluoroacetic, to the reaction mixture while using greater than therequired quantities of the amine base. The acids which may be used toform the salt include those which do not form a complex with the metalsalt catalyst or when employed the metal salt oxidant compoundsinactivating the catalyst and oxidant. As indicated hereinabove theacids must be of sufficient strength, i.e., stronger than water, andsuch that the anion will not complex with the metal catalyst or oxidantsalt. The salts which may be formed in situ may in themselves notnecessarily be isolable and may exist in equilibrium in the reactionmixture under carbonylation reaction conditions. Thus, such salts couldnot be added per se but, as indicated above may be formed in situ uponthe addition of a suitable acid to the reaction mixture containingamine.

Although not required, solvents, if desired, which are chemically inertto the components of the reaction system may be employed. Suitablesolvents include, for example, organic esters such as ethyl acetate,n-propyl formate, isopropyl acetate, sec- and iso-butyl acetate, amylacetate, cyclohexyl acetate, n-propyl benzoate, lower alkyl phthalates,etc. and the alkyl sulfones and sulfoxides such as propyl ethylsulfoxide, diisopropyl sulfone, diisooctyl sulfoxide, acetone,cyclohexanone, methyl formate, as well as N,N-dimethylformamide, etc.

As indicated above the reaction can be suitably performed by introducingthe oxygen and carbon monoxide at a desired pressure into contact withthe orthoborate ester reaction medium containing the specifiedreactants, catalyst, alcohol and amine and preferably an amine salt andoxidant salt and heating to the desired temperature. In general, acarbon monoxide pressure of about 500 psi to about 3000 psi partialpressure and preferably from 900 psi to about 2200 psi is employed.Stoichiometric quantities of carbon monoxide are generally employed.However, an excess of carbon monoxide may be employed, for example, incontinuous processes where a large excess of or high carbon monoxiderequirements are generally utilized, a suitable recycle of the carbonmonoxide may be employed. The reaction will proceed at temperatures offrom about 50° C. to 200° C. It is generally preferred to operate theprocess at temperatures in the range of 100° C. to 135° C. to obtain aconvenient rate of reaction. Heating and/or cooling means may beemployed interior and/or exterior of the reaction to maintain thetemperature within the desired range.

At least stoichiometric amounts of oxygen or an oxygen containing gassuch as air are generally employed and at any oxygen partial pressuresuch that the explosive range is avoided. Thus, the concentrations ofoxygen should be low enough so that the reaction mixture is notpotentially explosive. The Handbook of Chemistry and Physics, 48thEdition, 1967 indicates that the explosive limits of pure oxygen incarbon monoxide is 6.1 to 84.5 volume percent and air in carbon monoxideto be 25.8 to 87.5 volume percent.

The reaction time is generally dependent upon the orthoborate esterbeing reacted, temperature, pressure and on the amount and type ofcatalyst being charged as well as the type of equipment being employed.Reaction times will vary dependent on whether the process is continuousor batch.

The following examples are provided to illustrate the invention inaccordance with the principles of this invention but are not to beconstrued as limiting the invention in any way except as indicated bythe appended claims.

In the Examples which follow the reactions were run in a 300 mlstainless steel stirred autoclave. The liquid and solid materials werecharged to the reactor (as solutions whenever possible). At least 1000psi CO was charged to the reactor, which was heated to reactiontemperature. The pressure was increased to the desired value by addingmore CO. Oxygen was added in such an amount that a potentially explosivegas mixture was never obtained in the reactor. Enough CO was employed tosweep the oxygen out of the entire length of the tubing and into thereactor. The ensueing rate of gas uptake was allowed to level off beforethe next addition of CO or oxygen. Additional CO was charged to maintainconstant pressure. When an exotherm was observed, cold water wascirculated through the internal cooling coil to maintain the reactiontemperature within ±5° C. The process of charging oxygen and sweepingout the line with CO was repeated until no more gas uptake was observed.The reactor was cooled to ambient temperature. A gas sample wasobtained, and the composition was determined by mass spectral analysis.The liquid product was analyzed by gas-liquid phase chromatography (glc)for the oxalate and carbonate ester.

EXAMPLE I

A solution of 0.19 g. lithium iodide (1.41 mmoles), 2.34 g.triethylamine (23.0 mmoles), 6.96 g. triethylammonium sulfate (23.0mmoles), 50.7 g. methanol (1584 mmoles), and 55 ml trimethyl borate(0.53 mole) was charged to the autoclave along with 0.25 g. palladium(II) iodide (0.69 mmole) and 3.70 g. anhydrous copper (II) sulfate (23.0mmoles). The reaction temperature was 100° C. The total initial pressureat reaction temperature was 1500 psi. 100 psi oxygen was chargedfollowed by 200 psi CO. A rapid pressure drop was noted in addition to astrong exotherm. 200 psi CO was added to bring the total pressure backup above 1500 psi. The oxygen/CO charge cycle was repeated five moretimes. Total oxygen consumed was 634 psi. A total of 1600 psi pressuredrop was observed over a reaction time of approximately 3.8 hours. Thetotal pressure ranged between 1500 and 2125 psi during reaction Glcanalysis showed the presence of a boroxine and methanol but no trimethylborate was detectible. The liquid reaction product contained 0.29 moledimethyl oxalate and no dimethyl carbonate according to glc analysis.0.8 mole of CO₂ was detected in the gaseous product.

EXAMPLE II

The same amounts of materials as in Example I with the exception thatthe triethylammonium sulfate was replaced by 0.95 g. boric acid (15.4mmoles) were charged to the autoclave. The reaction temperature was 100°C. The total initial pressure was 1400 psi at reaction temperature. 65psi oxygen was charged followed by 200 psi CO. A strong exotherm andrapid pressure drop were noted. The oxygen/CO charge cycle was repeatedten more times. Total oxygen consumed was 1212 psi. A total of 2205 psipressure drop was observed during the reaction period of (7.0 hours).The total pressure ranged between 1400 and 2245 psi during the reaction.Glc analysis of the liquid showed the presence of methanol, anddimethoxymethane. Trimethyl borate was not detectible. The liquidreaction product contained 0.28 mole of dimethyl oxalate and a trace ofdimethyl carbonate according to quantitative glc analysis. 0.26 mole ofCO₂ was detected in the gaseous product.

EXAMPLE III

A solution of 2.34 g. triethylamine (23.0 mmoles), 6.96 g.triethylammonium sulfate (23.0 mmoles), 10.0 g. methanol (321 mmoles),and 55 ml trimethyl borate (0.616 mole) along with 0.65 mole ofN,N-dimethylformamide was charged to the autoclave. 0.25 g. palladium(II) iodide (0.69 mmoles), 0.19 g. lithium iodide (1.41 mmoles) and 3.70g. anhydrous copper (II) sulfate (23.0 mmoles were charged separately assolids. The reaction temperature was 125° C. The total initial pressureat reaction temperature was 1480 psi. 100 psi oxygen was chargedfollowed buy 200 psi CO. A rapid pressure drop was observed along with alarge exotherm. An additional 100 psi CO was charged in order to raisethe total pressure above 1500 psi. The oxygen/CO charge cycle wasrepeated five more times using between 200 and 300 psi CO after each 100psi charge of oxygen. A total of 2325 psi pressure drop was observedover a reaction time of approximately 5.8 hours. The total pressureranged between 1480 and 2050 psi during the reaction. The only productsdetectible by glc were dimethoxymethane, a boroxine, and dimethyloxalate. Some methanol and triethylamine were detected, but unreactedtrimethyl borate could not be detected. The liquid reaction productcontained 0.17 mole of dimethyl oxalate and no dimethyl carbonateaccording to quantitative glc analysis. 0.16 mole of CO₂ was detected inthe gaseous product.

EXAMPLE IV

A solution of 2.34 g. triethylamine (23.0 mmoles), 0.95 g. boric acid(15.3 mmoles), 3.0 g. ethanol (65 mmoles), and 60 ml triethyl borate(0.410 mole) and 0.387 mole of N,N-dimethylformamide was charged to theautoclave. The following solids were charged separately: 0.25 g.palladium (II) iodide (0.69 mmole), 0.19 g. lithium iodide (1.41mmoles), and 4.60 g. iron (III) sulfate (11.5 mmoles). The reactiontemperature and initial pressure were 100° C. and 1475 psi respectively.100 psi oxygen followed by 200 psi CO was charged. A total of 1825 psipressure drop was observed during a reaction period of 5.2 hours. Thetotal pressure ranged between 1475 and 2025 psi during the reaction.Total oxygen consumed was 717 psi. Analysis of the reaction productsshowed 0.13 mole of diethyl oxalate, a trace (< 0.005 mole) diethylcarbonate and 0.17 mole of CO₂.

EXAMPLE V

A solution of 2.34 g. triethylamine (23.0 mmoles), 3.0 g. methanol (93.6mmoles), and 55 ml trimethyl borate (0.616 mole) was charged to theautoclave along with 0.25 g. palladium (II) iodide (0.69 mmole), 0.19 g.lithium iodide (1.41 mmoles), 0.95 g. boric acid (15.4 mmoles), and 3.70g. copper (II) sulfate (23.0 mmoles), The reaction temperature was 100°C. The total initial pressure at reaction temperature was 1500 psi. 100psi oxygen was charged followed by 200 psi CO. An exotherm and rapidpressure drop were observed. The oxygen/CO charge cycle was repeated sixmore time using between 200 and 400 psi CO charges after each 100 psicharge of oxygen. A total of 2250 psi pressure drop was observed over areaction time of approximately 5.3 hours. The total pressure rangedbetween 1460 and 2000 psi during reaction. Total oxygen consumed was 800psi. Glc analysis showed the presence of a trace amount of dimethylcarbonate and 0.12 mole dimethyl oxalate. CO₂ (0.14 mole) was detectedin the gaseous reaction product.

EXAMPLE VI

A solution of 2.34 g. triethylamine (23.0 mmoles), 6.9 g. butanol (93.6mmoles), and 50 ml tri-n-butyl borate (0.218 mole) was charged to theautoclave. The solids charged were 0.25 g. palladium (II) iodide (0.69mmole), 1.05 g. triphenylphosphine (4.0 mmoles) and 3.70 g. anhydrouscopper (II) sulfate (23.0 mmoles). The reaction temperature was 110° C.The total initial pressure was 1500 psi. 100 psi oxygen was chargedfollowed by 200 psi CO. A relatively small exotherm and a moderate rateof pressure drop were noted. The next oxygen/CO charges showed exothermsand faster rates of pressure drop. 200-300 psi CO charges were usedafter each 100 psi oxygen charge. A total of 2330 psi pressure drop wasobserved over a reaction time period of approximately 5.8 hours. Thetotal pressure ranged between 1440 and 1865 psi during the reaction.According to quantitative glc analysis, the liquid reaction productcontained 26.1 g. dibutyl oxalate and 6.1 g. dibutyl carbonate. Thegaseous reaction product contained 1.58 g. CO₂.

EXAMPLE VII

A solution of 2.34 g. triethylamine (23.0 mmoles), 3.0 g. methanol (93.6mmoles), and 55 ml trimethyl borate (0.53 mole) was charged to theautoclave along with the separate solid materials: 0.18 g. palladium(II) chloride (1.0 mmole), 0.19 g. lithium iodide (1.41 mmoles), 3.69 g.copper (II) oxalate hemihydrate (23.0 mmoles), and 6.96 g.triethylammonium sulfate (23.0 mmoles). The reaction temperature was100° C. The total initial pressure was 1500 psi. 100 psi oxygen and 200psi CO were charged to the autoclave. A strong exotherm and rapidpressure drop were noted. The same results were obtained for the nextfour oxygen/CO charges in which 200-300 psi CO was charged after each100 psi oxygen charge. The sixth oxygen/CO charge showed no exotherm anda small pressure drop. The reaction mixture was maintained attemperature and pressure for 50 minutes after the last recorded pressuredrop. A total of 1725 psi pressure drop was observed over a reactiontime of approximately 4.6 hours. The total pressure ranged between 1425and 1850 psi. Dimethyl oxalate (0.361 mole) was obtained. Dimethylcarbonate was detected in the liquid product.

EXAMPLE VIII

A solution of 9.27 g. tributylamine (50.0 mmoles), 76.9 g. methanol (2.4moles), and 55 ml trimethyl borate (0.53 mole) along with 0.50 g.palladium (II) oxalate (2.6 mmole), 0.19 g. lithium iodide (1.41 mmoles)and 6.96 g. triethylammonium sulfate were charged to the autoclave. Thereaction temperature was 125° C. The total initial pressure was 1500 psiat reaction temperature. 100 psi oxygen was charged followed by 200 psiCO. No gas uptake or exotherm was detected up to 15 minutes afterward.When the oxygen/CO charge was repeated, immediately a strong exothermand rapid pressure drop were noted. The concentration of oxygen was suchthat the gas mixture was potentially explosive. Three more oxygen/COcharges were made, and again the reaction was initiated only whenpotentially explosive mixtures of oxygen and CO were employed. Thereaction was not run to completion. 1280 psi total pressure drop wasrecorded over a time period of approximately 7.7 hours. The totalpressure ranged between 1480 and 2100 psi during the reaction. Glcanalysis of the liquid product showed the presense of methanol, dimethylcarbonate, and unreacted trimethyl borate. The liquid contained dimethyloxalate (0.23 mole). The gaseous product contained CO₂ (0.264 mole).

EXAMPLE IX

A solution of 5.06 g. triethylamine (50 mmoles), 1.72 g. (16.9 mmoles)of concentrated sulfuric acid (96.4 percent), 3.0 g. methanol (93.6mmoles), and 55 ml trimethyl borate (0.53 mole) was charged to theautoclave along with 0.22 g. palladium (II) acetate (1.0 mmoles), 0.19g. lithium iodide (1.41 mmoles), 3.69 g. copper (II) oxalate hemihydrate(23 mmoles), and 1.58 g. oxalic acid dihydrate (12.5 mmoles). Thereaction temperature was 135° C. The total initial pressure at reactiontemperature was 1500 psi. Six 100 psi oxygen/200 psi CO charges wereadded to the autoclave over the period of 4.5 hours. Each addition ofoxygen/CO showed a relatively slow pressure drop and no noticeableexothermic behavior. A total pressure drop of 1150 psi was recorded. Thetotal pressure ranged between 1800 and 2240 psi during reaction. Glcanalysis of the reaction product showed the presence of methanol,unreacted trimethyl borate, dimethyl carbonate, and 20.9 g. dimethyloxalate (0.177 mole). 3.52 g. CO₂ (0.080 mole) was detected in thegaseous reaction product.

EXAMPLE X

A solution of 2.34 g. triethylamine (23.0 mmoles), 6.96 g.triethylammonium sulfate (23.0 mmoles), 1.4 g. ethanol (31.2 mmoles), 60ml triethyl borate (0.410 mole) was charged to the autoclave along with0.21 g. rhodium (III) chloride (1.0 mmoles) 0.19 g. lithium iodide (1.41mmoles), and 9.4 g. copper (II) trifluoroacetate (32.4 mmoles). Thereaction temperature was 100° C. The total initial pressure at reactiontemperature was 1500 psi. 100 psi oxygen followed by 200 psi CO wascharged. After the pressure levelled out, the oxygen/CO charge wasrepeated. A rapid uptake and strong exotherm were observed. Theoxygen/CO charge was repeated six more times. A total of 2125 psipressure drop was observed over a reaction time of approximately 6.8hours. The total pressure ranged between 1475 and 2050 psi duringreaction. Glc analysis of the recovered liquid product showed thepresence of methanol, 7.5 g. diethyl carbonate (0.064 mole), and 57.5 g.diethyl oxalate (0.394 mole). No triethyl borate was detectible in theliquid product. The gaseous product contained 15.0 g. CO₂ (0.341 mole).

EXAMPLE XI

A solution of 7.02 g. triethylamine (69.4 mmoles), 1.18 g. concentratedsulfuric acid (11.6 mmoles), 1.0 g. absolute methanol (31.2 mmoles), and55 ml trimethyl borate (0.53 mole) plus 0.25 g. palladium (II) sulfate(1.0 mmole), 0.19 g. lithium iodide (1.41 mmoles), and 3.70 g. anhydrouscopper (II) sulfate (23.0 mmoles) were charged to the autoclave. Thereaction temperature was 100° C. The total initial pressure at reactiomtemperature was 1500 psi. 100 psi oxygen was charged followed by 200 psiCO. Another oxygen/CO charge was made. A total of 680 psi pressure dropwas observed in a period of 2.7 hours. The reaction was not taken tocompletion. The total pressure during reaction was between 1420 and 1800psi. The gaseous reaction product contained 6.2 g. CO₂ (0.14 mole).According to glc analysis, the reaction product contained somemethanols, 16.3 g. dimethyl oxalate (0.138 mole), and 1.8 g. dimethylcarbonate (0.020 mole).

We claim
 1. A process for the preparation of oxalate esters whichcomprises reacting under substantially anhydrous conditions inorthoborate ester having the formula ##STR4## wherein R is alkyl oraralkyl groups which may contain substituents which do not interferewith the reaction, with carbon monoxide and oxygen at a pressure ofbetween about 500 psi and 3000 psi and at a temperature in the range ofabout 50° C. to 200° C. in the presence of an effective amount of acatalyst selected from the group consisting of palladium, platinum andrhodium salt compounds and a catalytic amount of an aliphatic,cycloaliphatic, aromatic or heterocyclic amine, and a catalytic amountof a monohydric aliphatic, alicyclic or aromatic alcohol which maycontain substituents which do not interfere with the reaction, andrecovering the desired oxalate ester.
 2. A process according to claim 1wherein the catalyst salt compound is selected from the group consistingof palladium, platinum and rhodium, halides, oxalates, sulfates andacetates.
 3. A process according to claim 2 wherein the catalyst isselected from palladium chloride, palladium iodide, palladium oxalate,palladium sulfate, palladium acetate, platinum chloride and rhodiumchloride.
 4. A process according to claim 3 wherein the catalyst ispalladium iodide.
 5. A process according to claim 1 wherein the amine isemployed in concentrations of from 0.1 to 5 weight percent.
 6. A processaccording to claim 5 wherein the amine is triethylamine.
 7. A processaccording to claim 1 wherein the alcohol is employed in concentrationsof from 0.1 to 50 weight percent.
 8. A process according to claim 7wherein the alcohol is selected from the group consisting of methylalcohol and ethyl alcohol.
 9. A process according to claim 8 wherein thealcohol is methyl alcohol.
 10. A process according to claim 1 whereinthe reaction is carried out in the presence of a catalytic amount of acopper (II) or iron (III) oxidant salt compound and an ammonium orsubstituted ammonium salt compound.
 11. A process according to claim 10wherein the oxidant salt compound is copper (II) or iron (III) oxalate,sulfate, acetate or trifluoroacetate.
 12. A process according to claim11 wherein the oxidant salt is copper (II) sulfate.
 13. A processaccording to claim 11 wherein the oxidant salt is copper (II) oxalate.14. A process according to claim 11 wherein the oxidant salt is iron(III) sulfate.
 15. A process according to claim 10 wherein the ammoniumsalt compound is triethylammonium sulfate.
 16. A process according toclaim 10 wherein the ammonium or substituted ammonium salt compound isformed in situ upon the addition of an acid to the reaction mixturecontaining an excess of amine over the required quantities of aminebase, said acid being of a strength stronger than water and such thatthe anion will not complex with the metal salt catalyst or metal oxidantsalt compound.
 17. A process according to claim 16 wherein said acid issulfuric acid.
 18. A process according to claim 16 wherein said acid isboric acid.
 19. A process according to claim 1 wherein the reaction iscarried out in the presence of a cocatalytic amount of an organic mono-or poly-dentate ligand or co-ordination complex of the metal catalystselected from the group consisting of alkyl, aryl and halogensubstituted phosphines, arsines, stibines and iodides.
 20. A processaccording to claim 19 wherein the ligand or co-ordination complex islithium iodide.
 21. A process according to claim 1 wherein the pressureis between about 900 psi and 2200 psi and the temperature is in therange of about 100° C. to 135° C.
 22. A process according to claim 21wherein the orthoborate ester is trimethyl borate, the catalyst ispalladium iodide, the amine is triethylamine and the alcohol is methylalcohol.
 23. A process according to claim 22 wherein a catalytic amountof a copper (II) sulfate, triethyl ammonium sulfate and lithium iodideis added to the reaction mixture.
 24. A process according to claim 21wherein the orthoborate ester is triethyl borate, the catalyst ispalladium iodide, the amine is triethylamine and the alcohol is ethylalcohol.
 25. A process according to claim 24 wherein a catalytic amountof a copper (II) sulfate, boric acid, and lithium iodide is added toreaction mixture.
 26. A process according to claim 1 wherein anoxygen-containing gas is employed as a source of oxygen for thereaction.
 27. A process according to claim 1 wherein the catalyst issupported.